Method for making float glass

ABSTRACT

In the production of high-quality flat glass by the float process, tridymite-frost stone defects are safely avoided by providing burners in the refiner zone of the furnace to shield the crown of that portion of the furnace from contact with alkali vapors, and at the same time providing not only a barrier that substantially separates the refiner-zone headspace from the melter headspace but also preferably a means associated with refiner-zone headspace for exhausting or venting it.

ilnited States Patent 119i Pecoraro et al.

[451 May 22, 1973 L54] METHOD FOR MAKING FLOAT GLASS [75] Inventors:George A. Pecoraro, Lower Burrell; John F. McConnell; Leonard A.

[73] Assignee: PPG Industries, Inc., Pittsburgh, Pa. [22] Filed: Apr.29, 1971 [2l] Appl. No.: 138,699

52 us. (:1 ..65/27, 65/99 A, 65/135, 65/136, 65/347 51 161. c1. ..C03b5/22 58 Field of Search ..65/27, 347, 135,

[56] References Cited UNITED STATES PATENTS 3,495,966 2/1970 West..65/137 Knavish, both of Pittsburgh, all of 3/1966 O'Connell et al...65/27 8/1970 Leveque ..65/135 Primary Exam inerRobert L. Lindsay, Jr.Att0rney-Chrisholm and Spencer 57] ABSTRACT In the production ofhigh-quality flat glass by the float process, tridymite-frost stonedefects are safely avoided by providing burners in the refiner zone ofthe furnace to shield the crown of that portion of the furnace fromcontact with alkali vapors, and at the same' time providing not only abarrier that substantially separates the refiner-zone headspace from themelter headspace but also preferably a means associated withrefiner-zone headspace for exhausting or venting it.

5 Claims, 1 Drawing Figure Patented GLASS IIIIVENTORS GEORGE A. PacoeuaJail/v E M: CONMELL some A. KA/AVISH ATTORNEYS METHOD FOR MAKING FLOATGLASS BACKGROUND OF THE INVENTION 1. Field of the Invention Thisinvention relates to production of high-quality flat glass by the floatprocess, and in particular, to the avoidance of tridymite-frost stonedefects in the prod uct of such process.

2. Description of Prior Art The production of high-quality flat glass bythe float process, such as that of U.S. Pat. No. 3,083,551, is practicedon a large scale. In that art, it is known that in order to obtain acommercially acceptable product, care must be exercised to eliminate, tothe greatest extent possible, the common kinds of defects that occur 15in such glass, such as seeds, blisters, ream knot and stones. Moreover,the measures taken toward the elimination of or the avoidance of suchdefects must be such that they produce no substantial deleterious effectupon the distortion quality of the product glass. For an operation to becommercially successful, it is generally necessary to produce glassexhibiting a defect density (considering defects of all known kinds) onthe order of 4.0 per 100 per square feet of glass, and preferably about2.0 or lower.

In the production of flat glass, it is known that one sort of defectoccasionally encountered is the tridymitefrost stone defectolt is knownthat this kind of defect results from the action of alkaline vapors uponmelting-furnace roof refractories, which commonly consist essentially ofsilica. The problem is of relatively little importance in the productionof plate glass, be cause the defects are usually removed by thesubsequent grinding and polishing operations. In a float-glass process,however, such grinding and polishing(which are costly steps) areavoided, and it becomes necessary to use other measures to avoiddifficulty with tridymitefrost stone defects.

It is known, moreover, that there is usually no problem with such stonesduring the first 8 to 12 months of use of a new or a newly-linedfurnace. Sometimes, the problem does not, even later, develop. If itdoes, various means are suggested in the prior art for dealing with it,but it is submitted that all of these have draw backs in comparison withthe instant invention.

In an article entitled, The Siliceous Scale Dropped From The TankFurnace Crown by N. Araki, it is suggested that the problem can beovercome by using a silica brick of especially low lime content. Thedrawback of this approach is the greater cost of such refractories.

In references such as British Pat. No. 1,067,006; British Pat. No.1,035,415; U.S. Pat. No. 3,240,581; and U.S. Pat. No. 3,238,030, it issuggested that the problem may be overcome by supplying to the furnace asodium-sulphur compound and a chemically reducing atmosphere. This has adrawback of requiring close control of process conditions.

In Canadian Pat. No. 851,103, it is suggested that the problem beovercome by reconstructing the furnace roof so that a constant bleedingof gases through the pores of the silica refractory may serve to protectthe refractory from attack by alkali vapors. Adoption of this expedientnecessitates a furnace shut-down for cold repair. It appearsquestionable, moreover, that the amount of gas permeating therefractories is sufficient to have the desired effect unless specialsilica refractories, more porous than those normally used, are employed,and at that, it is questionable whether there may be obtained a poroussilica-refractory material that has both the required porosity andsufficient strength to enable it to be used in a furnace-roof structureof substantial span.

In a patent application filed concurrently herewith in the name ofGeorge A. Pecoraro, one of the inventors herein Ser.. No. 138,822, thereis disclosed in practice of mechanically dislodging tridymite particlesfrom the roof of the refiner of the melting furnace. This solution tothe problem, while it has the advantage that it makes unnecessary anyday-to-day operating expense, suffers the drawback that it involveseither a loss of production (as, for example, when it is practicedduring a time that the product otherwise would be satisfactory) or thedrawback that it is sometimes necessary to tolerate a higher defectdensity in tridymite-frost stones than is desirable for a considerablyperiod of time, i.e., until there is a thickness change of dilutionchange or cold repair.

It is to be admitted that there are known, in processes for melting andrefining of glass, furnaces in which burners are provided in theforehearth or refiner section and/or a barrier is provided between theheadspace of the refiner zone and that of the melter, so that theheadspaces of the two zones comprise substantially differentatmospheres. In this regard, reference may be made to U.S. Pats. Nos.1,993,964; 2,600,490; and 2,767,235. The disclosures of the abovepatents lack, however, any appreciation of the importance of thesefeatures in connection with the problem of avoiding tridymite-froststone defects in the production of highquality flat glass by the floatprocess, and they lack any teaching of the combination of such featureswith an exhaust means or vent communicating with the refinerzoneheadspace, as taught herein.

SUMMARY OF THE INVENTION In the production of high-quality flat glass bythe float process, difficulty with tridymite-frost stone defects issafely avoided by providing burners in the front end of the furnace (inthe refiner zone) so that the silica crown of that portion of thefurnace is substantially kept from contact with alkali vapors, and atthe same time providing a barrier that separates the refiner-zoneheadspace from the melter headspace and preferably a means associatedwith the refiner-zone headspace for exhausting or venting it. By meansof the abovementioned combination of features, the development oftridymite-frost stone defect densities higher than are tolerable isindefinitely postponed, so that interruption of the process on accountof defects of that kind almost never becomes necessary. The benefits interms of production saved far outweigh the cost of the inventivemeasures adopted. The proposed inventive combination not only safeguardsagainst the development of the defects but also is such as to be adoptedwith a minimum of disturbance in the flow in the molten glass; moreover,it is also beneficial from the standpoint of tending to preclude thedevelopment in the glass in the refiner zone of the melting furnace ofRayleigh instabilities that tend to generate unwanted inhomogeneities(ream") in the product high-quality float glass.

DESCRIPTION OF THE DRAWING A complete understanding of the invention maybe obtained from the foregoing and following description thereof takentogether with the appended drawing, the sole FIGURE of which comprises aschematic elevation view of a portion of a glass-melting furnace thathas been provided with the features in accordance with the instantinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to the FIGURE, thereis shown a schematic indication of the downstream or refiner-end portion2 of a melting furnace having a shell 4, and in its lower portion, alining 6 of refractory material in contact with a bath 8 of moltenglass. The upper portion of the refiner portion 2 also as a refractorylining 10 that includes an arched roof portion 12. Those skilled in theart will appreciate that, in accordance with the prior art, the refinerportion 2 comprises a part of a large regenerative-type furnace for themelting of glass-batch materials, with the furnace being generallyrectangular in plan cross section and the depicted refiner portion 2comprising the portion, about 40 to 50 percent of the total length ofthe furnace, that is most distant from the end thereof at which theglass-batch materials are fed thereinto. To give an approximate idea ofthe scale of the operation, it is proper to say that the glass bath 8has a width on the order of 25 to 40 feet; that the overall length ofthe melting furnace, including the refiner zone 2, is on the order of150 to 400 feet; and that the amount of glass-batch material meltedtherein per day is on the order of 150 to 700 tons. In the furnace, theglass has a flow of the kind indicated by the arrows l4 and 16, i.e., inthe upper portions of the glass bath 8 which contains the hotter glass,there is a forward flow as indicated by the arrow 14, and in the coolerand lower portion of the bath 8, there is a return flow as indicated bythe arrow 16. As needed or desired, glass leaves the refiner portion 2of the melting furnace through the opening 18, passing then byconventional means, which are not shown but may include a flowregulatingtweel and a lip, onto a bath of molten tin and then therealong, underconditions of temperature, pressure, and applied force that adjustsuitably its viscosity and its thickness, to the exit end of themoltentin bath, at which the product flat glass is removed in the formof a sheet about N16 to 1 inch thick and 70 to 200 inches wide, forfurther processing.

All the features of apparatus described heretofore may be taken asconventional.

In accordance with the instant invention, a melting furnace having arefiner zone 2 as indicated above is provided with the additionalfeatures of a barrier 20 that separates the refiner-zone headspace 22from the melter-zone headspace 24; one or more burners 26 that projectthrough an opening or openings 28 in one or more of the walls 30 of thezone 2 of the melting furnace; and preferably but not necessarily also avent or exhaust means, generally indicated at 32. These features will bediscussed in greater detail hereinbelow.

The barrier 20 may take form of a drop arch or a shadow wall made ofsilica or other suitable refractory material. Preferably, it issubstantially imperforate and has a lower end 34 that is ideallypositioned at a distance of about 2 to 12 inches from the surface 36 ofthe glass bath 8. If the spacing is much larger than that, the desiredeffect of segregating the refiner-zone headspace 22 from the melter-zoneheadspace 24 can obtained only with the addition of large volumes oftemperature-controlled hot air, which is usually economicallydisadvantageous. With a smaller spacing, closer control of the level ofthe surface 36 becomes more important. Moreover, there is in someinstances inadequate room for the insertion of desired auxiliaryequipment such as a blown-gas homogenizer such as that of British Pat.No. 1,171,133. In principle, there is no reason why the spacing betweenthe end 34 of the barrier 20 and the surface 36 of the glass may not beless than 2 inches, being possibly as little as one-half inch.

It is frequently desirable for the barrier 20 to be positioned so thatis is located within a waist or channel that joins the melting zone andthe refining zone of the melting furnace. This makes possible a savingin the amount of material and time that is required for the constructionof the barrier 20, but it is not considered essential to the operationof the present invention. Such a waist or channel may have a width ofapproximately 10 to 60 percent of the overall width of a meltingfurnace.

Another feature comprising the combination of the instant inventionconsists in the use of one or more burners 26, introduced through anopening or openings 28 preferably in the front wall 30 of the refinerzone 2. An alternate location for the burners, but not one as desirable,is in the refiner breast wall, close to the front wall, with particularattention being given to the angle of burner insertion along with thevelocity of gases leaving the burner. As indicated in the drawing, theseproduce a non-luminous flame or flames 38. Actually, the non-luminousflame 38 may also be regarded merely as a jet of hot gases resultingfrom the combustion of, for example, natural gas and a sufficientproportion of excess air. It is quite important that luminosity beavoided because a luminous flame creates in the molten glass a patternof heating by radiation that engenders unwanted convective flows in theglass. Such convective flows may tend to deprive the glass of therelatively high degree of internal homogeneity that it should possess ifit is to be or remain useful for the making of high-quality flat glass.

The gases emanating from the burner or burners 26 serve to preventalkaline vapors from coming into contact with the arch portion or crown12 of the refiner zone 2. In this way, they serve to prevent thedevelopment on that refractory surface of particles of tridymite thatmight otherwise build up, over a period of months, and then begin tofall into the glass bath 8, appearing in the product as tridymite-froststone defects. It appears that for the most part, if there is anysimilar reaction between alkaline vapors and the roof of the melterportion of the melting furnace, either the reaction is less rapid or theparticles of tridymite that fall from the roof have an adequateopportunity to become dissolved in the glass, or both.

The refiner zone 2 stands nearer to the forming operation, and the glassin it is cooler, being at about 1,800 and 2,400 Fahrenheit. Inaccordance with the invention, it is considered essential that the gasesbe supplied through the burner or burners 26 at a rate sufficientlygreat to protect the crown l2 adequately. For a refiner zone 30 feetwide and approximately feet long, in the process of making float glassat a rate of about 350 tons per day, it must be considered necessarythat the air-gas volume be supplied at a rate of about 60,000 standardcubic feet per hour, though this may vary from 30,000 to 80,000 standardcubic feet per hour,

depending on conditions. Less will not reliably yield the desiredresults, and more is not 'only uneconomical from the standpoint of addedfuel consumption but also likely to have unwanted thermal effects uponthe molten glass. Although'for an installation operating on the scaleindicated above, satisfactory results may be obtained with a singleburner located near the'top of the crown l2 and having its outputdirected substantially horizontally, it is preferred that there be useda plurality of suitable burners, such as 2 to 6 of them, and possiblymore. These are arranged at suitably spaced intervals, with their outputbeing directed substantially horizontally and with them ideally but notnecessarily being located closer to the crown 12 than to the glasssurface 36. Satisfactory results may be obtained, however, with twoburner pipes, each projecting through a breast wall of the refiner'zoneand feeding gases having a composition and flow rate as indicatedelsewhere herein.

It is considered essential that the gases so introduced be hot, i.e.,within about 300 Fahrenheit of the temperature of the glass in the bath8, and preferably more closely matched that that to the temperature ofthe glass, i.e., within about 50 Fahrenheit. Where possible, it isdesirable that the gases be somewhat hotter than the glass, since thisreduces the tendency for the glass in the bath to develop Rayleighinstabilities that result in a product defect known as ream.

With respect to the composition of the materials introduced through themember or members 26, it may be stated that adequate results may beobtained with any hot gas that is substantially unreactive with the refractories comprising the crown 12. Thus, if a satisfactory supply ofhot air, nitrogen, or inert gas is available, such gas may be used. Inmost instances, however, it is least costly to produce the required gasby the combustion of a liquid or gaseous hydrocarbon fuel with asuitable quantity of gas containing at least about 21 percent of oxygen,e.g., excess air, such as 50 to 500 percent more than the stoichiometricamount required for combustion. The possibility of using air that hasbeen enriched with an addition of oxygen, to aid in obtaining desiredhigh flame temperatures is also not to be overlooked. It may bedesirable in some instances to preheat the air or oxygen-enriched airused to produce gases desired.

An example of suitable composition of gas comprising the flame or jet 38is the mixture resulting from the combustion of natural gas(substantially methane) with 50 percent of excess air, i.e., about 1mole of gas to 15 moles of air.

If flat glass of high quality is to be made, it is important to takeinto account the temperature and flow rate of the gas supplied, makingsure that the refiner zone 2 is not heated or cooledby too much. In thisregard, satisfactory results were obtained when operating on the scaleindicated above (refiner zone 30 feet by 75 feet, 350 tons per day) andwith the orifice 34 having an area of 16 square feet, by using a pair ofburners that have outlets located in the breast walls of the refinerzone on opposite sides of the refiner zone. The outlets weresubstantially flush with the breast wall, 2 feet above the metal lineand about the same distance from the front wall. The jets coming fromsuch outlets were angled about from the horizontal upwardly and about 10uptank. Each outlet provided combustion gases from 2,000 standard cubicfeet per hour of natural gas and 30,000 standard cubic feet per hour ofair.

The jets were issued from pipes 16 inches in diameter,

'but'of course, pipes much smaller could have been used. Thesatisfactory results thus obtained included long-term avoidance of thedevelopment of tridymitestone defects in the product high-quality floatglass, together with the avoidance of the development of unwanteddisturbances in the flows of the molten glass contained within therefiner zone, as evidenced by no.

detectableincrease in ream in the product glass attributable to thefront-end firing operation.

The invention also comprises in a preferred aspect the use of an exhaustor vent means 32 that communicates with the headspace 22 of the refinerzone 2 and serves to remove gases therefrom. This may take the form of asmokestack having its interior, at least in the bottom portion thereof,lined with refractory and being 20 or more feet high with suitablediameter when the invention is operated on the general scale indicatedabove. Such an exhaust or vent means may also comprise a damper means(not shown) for controlling or regulating (retarding) the rate ofpassage of gases therethrough, and/or a fan means (not shown) foraccelerating the passage of gases therethrough. It is consideredimportant that the exhaust or vent means, if used, be locatedsubstantially adjacent to the barrier 20. The vent may be permitted todischarge from the refiner directly into the melter under suitablepressure conditions.

The concept of providing a float line with the combination of featuresindicated above is particularly valuable in that it makes it possible toavoid the development of a situation in which, for example, the defectdensity in tridymite stones has risen to an undesirable level, such as0.5 or 0.7 per square feet of glass, yet no cold repair or thicknesschange or dilution change is scheduled for the near future. Prior to theinstant invention, it was considered necessary either to sacrificeproduction or to tolerate such a defect level, whereas the instantinvention serves to prevent the situation from arising. In anotheraspect, the combination of a barrier and the exhaust or vent means makesit possible to introduce in the refiner zone substantial quantities ofcrown-protecting hot gases, yet at the same time to avoid the upset inthe operation of the remainder of the melting furnace that might beencountered if these features are not employed. The refiner zone 2operates at a substantially lower temperature than the melter zone ofthe melting furnace, and if measures are not taken to permit the ventingor exhaust of the substantial quantities of the crown-protecting hotgases, it is to be expected that these gases, although hot, would passupstream in the melting furnace and then proceed to dilute thestill-hotter gases entering the checker chambers of the regenerativefiring system of the melter zone. Such dilution degrades the performanceof such checker chambers; they are less hot after the diluted gases passthrough them, and they accordingly are less able, when the regenerativefiring system is reversed, to provide the desired preheat to the airthat then passes through them.

Still another feature is that when the gases fed through the burner orburners 26 are hotter than the glass, there is a tendency to decreasethe depth below the glass surface 36 at which the customary temperatureinversion occurs, and as those who are familiar with the problems ofmelting high-quality flat glass and with advanced theories of fluidmechanics will readily appreciate, the depth of the location of saidtemperature inversion has a profound effect upon the Rayleigh Number ofthe fluid-flow system in the refiner zone 2. When the Rayleigh Number ofthat fluid-flow system is more than about 1,l00, the hotter and lessdense molten glass that is in the vicinity of the temperatureinversionlevel has a strong tendency to break loose, welling upward through themore dense and somewhat cooler glass above it, and this tends to cause aserious degeneration in the quality of the product flat glass. TheRayleigh Number is directly proportional to the cube of thetemperature-inversion depth. We have discovered that the cost ofsupplying gas to the burner or burners 26 is surprisingly small, incomparison with the cost of losing one or two days of production peryear as the result of defects in the nature of tridymite-frost stones orream of the Rayleigh instability type.

We claim as our invention:

1. In a process for making high-quality float glass wherein glass-batchmaterials are melted in a melting furnace laving a melter zone and arefiner zone, the improvement, whereby the development of unwantedtridymite-stone defects in the product glass is avoided, that comprisessegregating the headspace of said melter zone from the headspace of saidrefiner zone, and

providing to the headspace of said refiner zone at least onenon-luminous stream of hot gas in amount sufficient to shield the roofportion of said refiner zone from contact with alkali-containing vaporssaid hot gas having a temperature within approximately 300 Fahrenheit ofthat of the molten glass contained in the refiner zone.

2. An improvement as defined in claim 1, characterized by the furtherstep of continuously withdrawing, from the headspace of said refinerzone, gases contained therein through an exhaust means communicatingwith said refinerzone headspace and located downstream, with respect tothe glass flow, of means for segregating said melter-zone headspace fromsaid refiner-zone headspace.

3. A process as defined in claim 1, characterized in that said hot gaseshave a temperature greater than that of the molten glass in said refinerzone.

4. A process as defined in claim 1 characterized in that said hot gaseshave a temperature of about 1,800 Fahrenheit to 2,400 Fahrenheit andresult from the combustion of a hydrocarbon fuel with a gas containingat least about 21 per cent of oxygen.

5. A process as defined in claim 4, characterized in that said hot gasesresult from the combustion of natural gas with 50 to 500 percent ofexcess air.

1. In a process for making high-quality float glass wherein glass-batchmaterials are melted in a melting furnace laving a melter zone and arefiner zone, the improvement, whereby the development of unwantedtridymite-stone defects in the product glass is avoided, that comprisessegregating the headspace of said melter zone from the headspace of saidrefiner zone, and providing to the headspace of said refiner zone atleast one non-luminous stream of hot gas in amount sufficient to shieldthe roof portion of said refiner zone from contact with alkalicontainingvapors said hot gas having a temperature within approximately 300*Fahrenheit of that of the molten glass contained in the refiner zone. 2.An improvement as defined in claim 1, characterized by the further stepof continuously withdrawing, from the headspace of said refiner zone,gases contained therein through an exhaust means communicating with saidrefiner-zone headspace and located downstream, with respect to the glassflow, of means for segregating said melter-zone headspace from saidrefiner-zone headspace.
 3. A process as defined in claim 1,characterized in that said hot gases have a temperature greater thanthat of the molten glass in said refiner zone.
 4. A process as definedin claim 1 characterized in that said hot gases have a temperature ofabout 1,800 Fahrenheit to 2, 400* Fahrenheit and result from thecombustion of a hydrocarbon fuel with a gas containing at least about 21per cent of oxygen.
 5. A process as defined in claim 4, characterized inthat said hot gases result from the combustion of natural gas with 50 to500 percent of excess air.